In recent years, as electronic elements, many elements generating a large quantity of heat when used have come to be used, examples thereof including light emitting didoes (also abbreviated to LEDs hereinafter), organic electroluminescence (also abbreviated to organic EL hereinafter) elements, and power semiconductors. Such elements, which are large in generated-heat-quantity, have problems that generated heat deteriorates the element themselves, and deteriorates members in the vicinity of the elements. Thus, it is general to use a substrate excellent in heat dissipating performance as a substrate on which electronic elements as described above are arranged.
As such a substrate, which has heat dissipating performance, known is a heat dissipating substrate having a support base material, an insulating layer containing an insulating resin bonded onto the support base material, and a wiring layer formed on the insulating layer and connected electrically to electronic elements arranged on this wiring layer, that is, a heat dissipating substrate.
Although epoxy resin or thermoplastic polyimide resin used as the insulating resin has an advantage of being easily bonded onto the support base material, the resin has problems of being insufficient in insulating performance, heat resistance and others so as not to be easily made into a thin film, of being difficult to reduce short circuits in a heat dissipating substrate, and of making difficult the heat dissipating substrate be excellent in heat dissipating performance.
The resin also tends to be higher in coefficient of linear thermal expansion than the support base material to have a problem that the resin is warped, peeled from the support base material, or is subjected to other damages.
In order to solve such problems, for example, Patent Literatures 1 to 3 (Japanese Patent Application Publication (JP-A) No. 2003-071982, JP-A No. H09-099518, and JP-A No. 2004-230670) each discloses, as an insulating layer as described above, a layer having a basic insulating layer and adhesive layers made of thermoplastic polyimide resin and formed on both surfaces of the basic insulating layer, respectively.
According to such inventions, the use of the basic insulating layer made of material high in insulating performance makes it possible to prevent the generation of short circuit stably.
However, it is difficult to make the support base material, the basic insulating layer, and the adhesive layers made of thermoplastic polyimide resin equivalent to one another in coefficient of linear thermal expansion. Thus, there is caused a problem that the substrate cannot be sufficiently restrained from being warped, nor undergoing peel therein or other damages when heated. There also arises a problem that the substrate includes the adhesive layers made of thermoplastic polyimide resin so that the insulating layer becomes large in thickness not to exhibit a sufficient heat dissipating performance.